1. Field of the Invention
The present invention relates to an output circuit, the output of one end of which has high impedance and, more particularly, to an output circuit which stably operates when a power supply voltage is low. The present invention further relates to a temperature switch IC and a battery pack, each of which is provided with the aforementioned output circuit.
2. Description of the Related Art
The following will describe a conventional output circuit. FIG. 7 is a circuit diagram illustrating a conventional output circuit.
The conventional output circuit includes an inverter 97 connected to an input terminal, an NMOS transistor 93, which is an output driver, a diode-connected NMOS transistor 95 and a capacitor 96, which are provided between a power source and the ground, and an NMOS transistor 94 controlled thereby.
When the circuit is turned on, a power supply voltage VDD gradually rises. The NMOS transistor 95 remains nonconductive while the power supply voltage VDD is lower than a threshold voltage Vthn 95. The NMOS transistor 94 turns off, because the gate voltage thereof becomes an earth voltage VSS due to the capacitor 96. Hence, the output terminal of the output circuit is in a high-impedance state. This ensures that the output terminal of the output circuit is always set in the high-impedance state if the power supply voltage VDD at the time of, for example, turning the circuit on, is lower than a minimum operating voltage of the circuit.
When the power supply voltage VDD exceeds the threshold voltage Vthn 95 of the NMOS transistor 95, the NMOS transistor 95 becomes conductive. The capacitor 96 is charged by the current supplied by the NMOS transistor 95. When the gate voltage thereof gradually rises and exceeds a threshold voltage, the NMOS transistor 94 turns on. When the NMOS transistor 94 turns on, the function of the NMOS transistor 93 is rendered valid, transmitting an output of the inverter 97 to the output terminal. If the voltage of an input terminal of the output circuit is at a low level, then the NMOS transistor 93 turns on, and an output voltage VOUT of the output terminal becomes the earth voltage VSS. If the voltage at the input terminal of the output circuit is at a high level, then the NMOS transistor 93 turns off, causing the output voltage VOUT of the output terminal to be set in the high-impedance state (refer to, for example, patent document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 06-075668
In the conventional output circuit, the NMOS transistor 94 is provided in series with the NMOS transistor 93. The NMOS transistor 93, which is an output driver, is required to provide a drive capability. For this reason, a large NMOS transistor is used for the transistor 93. Thus, the NMOS transistor 94 is required to provide a drive capacity that is equivalent to or higher than that of the NMOS transistor 93.
The conventional output circuit has been posing a problem that the large size of the NMOS transistor 94 inconveniently leads to a large area of the output circuit.